An IR spectrum shows a strong, broad absorption around $3300–3500 \mathrm{cm}-1$ and a sharp absorption near $1700 \mathrm{cm}-1$. Which functional group is most likely present?

• A. Carboxylic acid
• B. Alcohol
• C. Alkene
• D. Amine

Answer: (A)

Verified step by step guidance

1. Identify the significance of the broad absorption around 3300–3500 cm\textsuperscript{−1} in the IR spectrum. This region typically corresponds to O–H or N–H stretching vibrations, which are characteristic of alcohols, amines, or carboxylic acids.
2. Note the sharp absorption near 1700 cm\textsuperscript{−1}, which is indicative of a C=O (carbonyl) stretching vibration. This peak is common in aldehydes, ketones, carboxylic acids, esters, and amides.
3. Combine the information from both absorptions: a broad O–H stretch and a sharp C=O stretch suggest the presence of a functional group containing both these features, such as a carboxylic acid.
4. Recall that alcohols show a broad O–H stretch but lack the strong C=O stretch near 1700 cm\textsuperscript{−1}, while amines have N–H stretches but no carbonyl peak, and alkenes show C=C stretches around 1600 cm\textsuperscript{−1} rather than 1700 cm\textsuperscript{−1}.
5. Conclude that the combination of a broad O–H stretch and a sharp C=O stretch near 1700 cm\textsuperscript{−1} is characteristic of a carboxylic acid functional group.